The present invention relates to a low-loss microwave device test fixture.
Low-loss microwave device test fixtures are typically used in automatic and manual RF/microwave (200 MHz to 26.5 GHz) measurement and testing of microstrip and coplanar devices (hereinafter xe2x80x9cDUTxe2x80x9d) operating in linear or non-linear mode.
These test fixtures are used within measurement set-ups to provide a mechanical base on which to physically install a RF/microwave device to be measured, such as transistor, diode, MMIC, etc. and provide a means to create a signal path from the inputs and the outputs of the devices to measurement equipment. The use of a test fixture is dictated by the fact that the packages in which devices are delivered are meant for PC-board mounting applications and cannot be hooked directly on test equipment connectors.
Generally, the packages in which the devices 100, 100xe2x80x2 are delivered comprise a main body 103, 103xe2x80x2, usually meant to be secured to a circuit ground, and two or more flanges 101, 101xe2x80x2, identified as inputs, outputs and sometimes grounds as shown in FIGS. 7a and 7b. All these elements are internally connected to different sections of the actual microwave microchip die. In all existing fixtures, devices are positioned and retained in place through a variety of methods (soldering, clamping, spring loading, etc.), and can be adjusted to different packages by replacing, adding or removing mechanical sections and PC-boards optimized for each specific application.
In almost all prior art fixtures, the transitions between the fixture connectors and the device package flanges are realised by using printed copper patterns over some dielectric substrate. The reason for this is that such an arrangement permits easy adaptation to different packages just by replacing a PC-board, as well as the possibility of achieving impedance transformation, consequently allowing the characterization of very low impedance devices. Also, each transition can be easily designed for the specific package dimensional characteristics (flanges of different sizes, placed at various heights, etc.). In particular, this allows the characterisation of devices mounted within geometrically asymmetrical packages. Unfortunately, such an approach also has a major draw-back: all known dielectric materials used in substrates have high ohmic losses compared to air.
In the very case in which devices presenting extremely low impedances at one of their ports have to be tested using a Load Pull Test System, which varies the load or source impedance seen by the devices at their ports, the ohmic losses generated by the dielectric material cannot be ignored, effectively putting a limit to the lowest possible impedance that can be accurately generated by the load pull test system. In fact, non-resistive losses can always be extracted mathematically from the values measured (by a process known as xe2x80x9cde-embeddingxe2x80x9d). However, it is of no importance to know exactly the effect of the ohmic losses in order to create the appropriate test conditions for the device; it is the fact that these ohmic losses limit the lowest available impedance presented to the device that matters. The most obvious approach to solve this problem is to design a fixture presenting as little losses as possible along the signal path. This is done by using dielectrics presenting as little ohmic losses as possible, the most appropriate dielectric for this purpose being empty space, closely followed by air.
However, up to now, air has been used as a dielectric in commercially available microwave device test fixtures only once. This test fixture is illustrated in FIGS. 4 and 5, identified as xe2x80x9cPrior Artxe2x80x9d. The test fixture has conductors having flange receiving portions. A section at the bottom of the device is custom-made for each separate DUT that is to be tested with the test fixture. Accordingly, whenever a new DUT is to be tested, the custom section must be replaced. Furthermore, this test fixture includes pressure columns (shown in solid in FIGS. 4, 4a and 4b) in isolating material to maintain the DUT signal flanges in place, which by definition introduces losses; additionally, pressure columns are provided, again in isolating material, to hold the DUT ground flanges in place (see FIG. 5).
This specific test fixture is inherently limited to small, low-power devices. Another limitation of this test fixture is that it does not provide any impedance transformation with the same custom section.
It is known from theory that achieving effective transitions using air as a dielectric and presenting good impedance transformation characteristics (more than 5:1) involves transitions of an extremely large size. Recent progress in measurement equipment and technologies however permits the direct measurement of extremely low impedances over very large frequency ranges. Consequently, investigating an approach using air has become far more interesting.
It is an object of the invention to provide a microwave test fixture which presents as little losses as possible along the signal path, and which can permit impedance transformations. In accordance with the invention, this object is achieved with a test fixture which said device has at least two flanges lying on an axis and projecting on opposite directions from the device, and at least one ground flange, said test fixture comprising:
a frame having two opposite extremities, a top, a bottom, a height, and a width;
a device supporting column located between said opposite extremities, having a top surface lying below said top of said frame and adapted to receive said device, said column having a height less than said height of said frame;
two adjustable blocks, each located between the device supporting column and an opposite extremity, said blocks being vertically adjustable;
two brackets for receiving a flange from the device, the brackets lying above the column and the blocks, each bracket being secured to an opposite extremity of the frame;
means for securing said device to said column; and means for securing said column and said blocks at a given vertical position.
The advantages provided by the microwave test fixture according to the present invention are the following: the fixture uses air as the exclusive dielectric transmission media; the characteristic impedance may be varied by continuously changing the distance of the signal carrying conductor from the ground plane; and the fixture can inherently be securely attached to a wide variety of low and high power microwave transistor packages in order to make their testing possible.